1. Field of the Invention
This invention relates to termination circuitry for high speed digital transmission lines and more particularly to RC circuitry for terminating such transmission lines and methods for producing such circuitry.
2. Brief Description of the Prior Art
As higher and higher frequencies have come to be employed in digital electronic equipment, such as computers and the like, the effects of signal travel time between the components of the circuits have had to be considered in the design of the equipment. When the frequency of a digital electrical signal within such equipment is so high that the rise and/or fall times of the signal are shorter than the travel time of the signal from source to destination within the equipment, the signal pathways have to be treated as transmission lines. In such circuits, if the rise/fall time of the signal is shorter than about twice the propagation time from source to terminal, reflections and other signal distortions can occur. Such reflections and the like can cause switching errors, among other problems. Accordingly, good design requires that they be eliminated or at least minimized.
Reflections occurring in transmission lines are conventionally suppressed by proper termination of the transmission line. Depending upon the type of connection between the source or driver and the load connected at the terminus of the transmission line, different conventional termination types have been devised, e.g., parallel, AC, series, and Thevenin. A convenient termination circuit is parallel termination, achieved by connecting the terminal end of the line to ground through an impedance equal to the characteristic impedance of the transmission line. A simple resistor of appropriate value connected between the line terminus and ground can provide a satisfactory termination. However, such a circuit has the disadvantage that it will draw DC power. Accordingly, a termination circuit comprising a resistor in series with a capacitor is often used. In order to adequately suppress reflections, the time constant of the RC circuit should be greater than the propagation time of a signal over the transmission line. A general criterion for assuring that the time constant will be adequate is given byCLR0>25tf  (1)where:                CL=terminating capacitance (F);        R0=terminating resistance (Ω);        tf=propagation time (time of flight).        
The value of the capacitor is not necessarily critical for most applications. Such applications typically include termination of microstrip transmission lines and stripline transmission lines used as circuit elements in printed wiring boards (PWBs) and multichip modules (MCMs). A value of about 50–100 pF has been reported as a good compromise for such applications. A value of 200 pF yields better signal quality, but allows greater power dissipation. Further increase in the capacitance ordinarily yields no significant improvement in signal quality and further increases the power dissipation. A lower value for the capacitor tends to make the termination ineffective.
Transmission line structures have been used for some time for routing signals between components in high-frequency circuitry formed on insulating, generally planar substrates, so-called “printed wiring boards” (PWB). Integrating functional passive devices, such as resistors and/or capacitors, into such PWB circuitry is a relatively recent development. Similarly, forming capacitor and resistor layers on either a printed wiring board or a substrate for high-density interconnection of silicon multichip modules (MCM) is also relatively new.
One commercially available structure has been developed by the AVX Corporation. In such circuitry, resistance and capacitance are distributed through a multilayer hybrid structure in which the resistive elements are made of ruthenium oxide (RuO2). This oxide is used because it has both higher conductivity and greater stability than other oxide conductive materials. Consequently, resistors can be formed having values of about 50–100 Ω. The dielectric in such circuits is comprised of a glass-loaded ceramic material, whereby the mechanical and thermal properties of the resistor and capacitor elements are well matched, a circumstance which leads to good reliability. These structures are prepared by conventional thick film techniques, wherein successive patterned layers of conductive, dielectric and resistive material are deposited on a typically ceramic substrate by silk screen techniques and subsequently fired. Connecting the passive devices in the layers in parallel permits reducing the resistance to a level suitable for impedance line matching, i.e., from about 10 Ω to about 220 Ω. Simultaneously, the capacitance can be raised to a useful range, i.e., about 10–220 pF. This technology permits the formation of passive components in high yield to a tolerance of about 10%. However, the use of the thick film manufacturing process limits the technique to preparation of ceramic-supported structures or surface mount packages to be placed on a circuit board.
Thin-film capacitors have been prepared by depositing a metal on a substrate to serve as one plate of a capacitor, subsequently depositing a dielectric layer thereon, then depositing a second metal layer to serve as the other plate of the capacitor. The dielectric layers for such capacitors have been deposited by conventional procedures such as sputtering, sol-gel processing, metallo-organic chemical vapor deposition (MOCVD), plasma enhanced chemical vapor deposition (PECVD), pulse laser deposition, and the like. Oxide dielectrics have also been prepared by electrochemical anodization, and oxide layers so prepared have been shown to perform better than oxide layers formed by reactive sputtering. However, no integrated process appears to have been disclosed in which a thin-film resistor and capacitor have been formed as a unitary RC circuit suitable for use as a transmission line termination.
Accordingly, a need has continued to exist for a method of preparing passive RC circuits suitable for use as transmission line terminations and the like.